Page 505 - Carrahers_Polymer_Chemistry,_Eighth_Edition

468 Carraher’s Polymer Chemistry

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Instantaneous polarization occurs when rapid (< 10 s) transitions occur, that is, at frequencies
10
greater than 10 Hz or at wavelengths less than 1 cm. Electronic polarization falls within this cate-
gory and is due to the displacement of charges within the atoms. Electronic polarization is directly
proportional to the number of bound electrons in a unit volume and inversely proportional to the
forces binding these electrons to the nuclei of the atoms.
Electronic polarization occurs so rapidly that there is no observable effect of time or frequency
on the dielectric constant until frequencies are reached that correspond to the visible and UV spec-
tra. For convenience, the frequency range of the infrared through the UV region is called the optical
frequency range, and the radio and the audio range is called the electric frequency range. Electronic
polarization is an additive property dependent on the atomic bonds. Thus, the electronic polariza-
tions and related properties are similar for both small molecules and polymers. Accordingly, values
obtained for small molecules can be applied to analogous polymeric materials. This does not apply
when the polymeric nature of the material plays an additional role in the conductance of electric
charges, as in the case for whole-chain resonance or whole-chain delocalization of electrons.
Atomic polarization contributes to the relative motion of atoms in the molecule affected by per-
turbation by the applied fi eld of the vibrations of atoms and ions having a characteristic resonance
frequency in the infrared region. The atomic polarization is large in inorganic materials, which
contain low-energy-conductive bonds and approaches zero for nonconductive polymers. The atomic
polarization is rapid, and this, as well as the electronic polarization, constitutes the instantaneous
polarization components.
The remaining types of polarization are absorptive types with characteristic relaxation times
corresponding to relaxation frequencies. Debye, in 1912, suggested that the high dielectric constants
of water, ethanol, and other highly polar molecules was due to the presence of permanent dipoles
within each individual molecule and that there is a tendency for the molecules to align themselves
with their dipole axes in the direction of the applied field. The major contributions to dipole polar-

izations are additive and are similar whether the moiety is within a small or large (macromolecule)
molecule. Even so, the secondary contributions to the overall dipole polarization of a sample are

dependent on both the chemical and physical environment of the specific dipole unit, its size, and its
mobility. Thus, dipole contributions can be used to measure the T and T .
g m
The polarizations noted above are the major types found in homogeneous materials. Other types
of polarization, called interfacial polarizations, are the result of heterogeneity. Ceramics, polymers
with additives and paper are considered to be electrically heterogeneous.
Table 13.3 contains often used electrical units.

TABLE 13.3
Selected Electrical Primary and Derived Units
SI Units
Electrical Value Symbol Primary Derived
2
Capacitance C s -C /kg-m 2 farad
2
2
Conductivity s s-C /kg-m 3 1/ohm-meter
Dielectric constant e, e r Simple ratio with no units
2
Dielectric displacement D C/m 2 farad-volt/m
Electric charge Q C coulomb
Electrical current I C/s ampere
2
Electric polarization P C/m 2 farad-volt/m
2
2
Electric potential V kg-m /s -C volt
2
Permittivity e s C /kg-m 3 farad/meter
2
2
Resistance R kg-m /s-C 2 ohm
3
Resistivity r kg-m /s-C 2 ohm-meter

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